Field of the Invention
The present invention relates to an optical transmitter.
Related Background of the Invention
Recently, for speed-up and capacity increase of a communication network, miniaturization and power consumption reduction of an optical transceiver (including both optical transmitter and optical receiver) have been continuously demanded from an optical communication system market. For example, as a 100-gigabit optical transceiver, CFP (100G Form-factor Pluggable) products have started to spread in the market. Also, lately, study of industrial standards called MSA (Multi Source Agreement) such as CFP2 and CFP4 of the next generation is underway for further miniaturization by a factor of one half and one quarter from the CFP.
When miniaturizing (highly integrating) an optical transceiver, it is preferable that internal components and an IC can be connected through a serial communication bus with a small number of signal lines. For example, in Patent Literature 1, plurality of internal components like a PHY (Physical Layer Chip) to communicate in a physical layer by electric signals, a CPU (Central Processing Unit) to monitor and control inside of the optical transceiver, a memory and so on, are interconnected through an internal serial communication bus. For the optical transceivers such as the CFP products, for example, two optical transceivers are connected to each other through a duplex optical fiber for bidirectional communication by optical signals. Therefore, an optical transmitter (optical transmitting unit) to convert an electric signals to an optical signal and transmit the converted optical signal outward and an optical receiver (optical reception unit) to receive an optical signal and convert the received optical signal to an electric signal are included in an optical transceiver.
Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2008-113207
The optical transmitter described above has two modes, one of which is a power saving mode for suppressing the entire power consumption as much as possible, practically less than a value specified in the MSA standard, and another of which is an operating mode for normally transmitting an optical signal. For example, for the CFP products, the power consumption should be smaller than 1 W for the power saving mode, but may be larger than 8 W for the operating mode. In the power saving mode, it is needed to run only the minimum parts such as an MCU (Micro Controller Unit) including a communication circuit with an external device (host system) of an upper layer and an arithmetic processing circuit, and to shut off an internal power supply to the other components such as a signal processing circuit for transmission and reception of optical signals. At the time, when the internal power supply for the other components is stopped and one of terminals of a component connected to the serial communication bus is pulled down to Low level of logic signal levels, the communication among all the components connected through the serial communication bus becomes unavailable because the serial communication bus is stuck
An optical transmitter is required to monitor a temperature inside the optical transceiver and answer the monitored temperature to the host system for monitoring and control in the upper layer. Hereinafter, the temperature inside the optical transceiver is called “internal temperature”. The internal temperature corresponds to an environmental temperature common for a plurality of internal components, and is different from a temperature inside a specific component (for example, an optical transmitter module). As a method of monitoring the internal temperature, an analog circuit to monitor resistance of a thermistor, which changes with the temperature, is used. Also, as another method, a prescribed component with a temperature detection circuit, which detects the temperature and outputs data of the detected temperature through the serial communication bus is used.
However, the analog circuit needs a large mounting area because a lot of electrical parts are included. Therefore, from a viewpoint of the miniaturization of the optical transceiver, the method of using the prescribed component and communicating with it through the serial communication bus is preferable because the prescribed component is available as an IC with a small package. However, as described above, since the serial communication bus is stuck in the power saving mode, it becomes difficult to monitor the internal temperature through the serial communication bus.